JP2020100861A - Machine component for automobiles made of steel material for induction hardening excellent in static torsional strength and torsional fatigue strength - Google Patents

Abstract

To provide a machine component for automobiles made of a steel material for induction hardening, excellent in static torsional strength and torsional fatigue strength.SOLUTION: Provided is a machine component for automobiles made of a steel material for induction hardening that contains, by mass%, C:0.42 to 0.48%, Si:0.20 to 1.10%, Mn:0.70 to 0.90%, P:0.030% or less, S:0.030% or less, Cr:0.90 to 1.20%, Al:0.010 to 0.040% and N:0.0200% or less, and the balance Fe with inevitable impurities. The machine component for automobiles has the following values in an induction hardened/tempered state: the surface hardness is 600 Hv or more, the core hardness is 350 Hv or more, the ratio of (the hardened layer depth/the component radius) is 0.5 to 1.0, the cross-sectional average hardness as expressed in the formula below is 550 Hv or more, and the grain size number of the outermost surface is 7.0 or more.SELECTED DRAWING: Figure 1

Description

TECHNICAL FIELD The present invention relates to a steel material for induction hardening which is excellent in static torsional strength and torsional fatigue strength, and more particularly to a mechanical part for an automobile such as an automobile shaft member.

2. Description of the Related Art In recent years, high torque and high output have been advanced in automobile engines. Along with this, there is a demand for higher strength of a shaft member that is a shaft component that outputs an input torque from an engine. Further, since a torsion stress is applied to the shaft member, static torsion strength and torsion fatigue strength are required.
Therefore, as the conventional shaft member, medium carbon alloy steel such as SCr440 or SCM440 has been used. However, along with the background of high torque and high output of automobiles as described above, it is required to develop mechanical parts for automobiles such as shaft members which are further excellent in static torsional strength and torsional fatigue strength.

Therefore, by specifying the structural area ratio of ferrite before induction hardening with respect to the carbon content, the ferrite crystal grain size, the width of the ferrite band, the aspect ratio of MnS, etc., an application relating to steel and parts having excellent torsional fatigue properties has been filed. It has been proposed (for example, refer to Patent Document 1). However, if specified from the above viewpoint, there is a problem that the operating conditions in steel making and rolling processes are restricted.

Therefore, by specifying the microstructure before induction hardening, the ratio of the effective hardened layer to the radius after induction hardening, the grain size, the average hardness of the cross-section, etc., an application for a steel material with excellent torsional strength and its parts has been proposed. (For example, see Patent Document 2).
However, the microstructure before induction hardening is a pearlite-based steel with a pro-eutectoid ferrite volume ratio of less than 15%, the core of this steel after induction hardening and tempering is soft, and the effective hardened layer/radius ratio is 0. Since it is 4 to 0.6, it is not possible to expect a significant increase in the average cross-sectional hardness effective for improving the torsional strength.

JP, 2002-069566, A JP, 2010-189702, A

Therefore, the problem to be solved by the present invention is to provide a mechanical part for automobiles made of induction hardening steel, such as a shaft member, which is induction hardened/tempered and is excellent in static torsional strength and torsional fatigue strength. ..
Further, after mechanical induction hardening/tempering of an automobile mechanical part made of steel for induction hardening, a shot peening layer is further added, which has excellent static torsional strength and torsional fatigue strength. Is to provide.

なお、ａ：捩り試験片の半径、Ｈｖ（ｒ）：ビッカース硬さ、ｒ：中心からの距離である。 Means for solving the above-mentioned problems is, in the first means, as chemical components, in mass%, C: 0.42 to 0.48%, Si: 0.20 to 1.10%, Mn:0. .70 to 0.90%, P: 0.030% or less, S: 0.030% or less, Cr: 0.90 to 1.20%, Al: 0.010 to 0.040%, N: 0. A machine part for an automobile, which contains 0200% or less and the balance is induction hardening steel consisting of Fe and unavoidable impurities,
In the induction-hardened/tempered state, the surface hardness is 600 Hv or more, the core hardness is 350 Hv or more, the ratio of (hardened layer depth/part radius) is 0.5 to 1.0, and is expressed by the following mathematical formula. An automotive machine part having an average cross-section hardness of 550 Hv or more and a grain size number of the outermost surface of 7.0 or more.

Note that a is the radius of the torsion test piece, Hv(r) is the Vickers hardness, and r is the distance from the center.

なお、ａ：捩り試験片の半径、Ｈｖ（ｒ）：ビッカース硬さ、ｒ：中心からの距離である。 In the second means, in addition to the chemical components described in the first means, from the steel for induction hardening containing Mo: 0.10 to 0.30% by mass% and the balance Fe and unavoidable impurities. Which is a machine part for automobiles,
In the induction-hardened/tempered state, the surface hardness is 600 Hv or more, the core hardness is 350 Hv or more, the ratio of (hardened layer depth/part radius) is 0.5 to 1.0, and is expressed by the following mathematical formula. An automotive machine part having an average cross-section hardness of 550 Hv or more and a grain size number of the outermost surface of 7.0 or more.

Note that a is the radius of the torsion test piece, Hv(r) is the Vickers hardness, and r is the distance from the center.

なお、ａ：捩り試験片の半径、Ｈｖ（ｒ）：ビッカース硬さ、ｒ：中心からの距離である。 In the third means, in addition to the chemical components described in the first or second means, Ti: 0.010 to 0.050% and B: 0.0003 to 0.0030% are contained by mass %. A machine part for an automobile made of induction hardening steel, the balance of which is Fe and unavoidable impurities,
In the induction-hardened/tempered state, the surface hardness is 600 Hv or more, the core hardness is 350 Hv or more, the ratio of (hardened layer depth/part radius) is 0.5 to 1.0, and is represented by the following mathematical formula. An automotive machine part having an average cross-section hardness of 550 Hv or more and a grain size number of the outermost surface of 7.0 or more.

Note that a is the radius of the torsion test piece, Hv(r) is the Vickers hardness, and r is the distance from the center.

In the fourth means, the mechanical part for automobile has a shot peening layer on the mechanical part for automobile which is induction hardened/tempered, and the surface hardness of this shot peening layer is 700 Hv or more and the compressive residual stress on the surface. Has a value of 1000 MPa or more. The automotive machine component according to any one of the first to third means.

In the fifth means, the vehicle mechanical component is the vehicle mechanical component according to any one of the first to fourth means, which is a shaft member.

According to the present invention, when the induction hardening steel having the component composition defined in the present invention is subjected to quenching/tempering with a high frequency of 2 to 30 KHz which has been conventionally used in automobiles and other industrial machines, it is used for automobiles. The mechanical component has a surface hardness of 600 Hv or more, a core hardness of 350 Hv or more, a hardening layer depth/radius ratio of 0.5 to 1.0, a cross-section average hardness of 550 Hv or more, and an outermost surface crystal. Since a steel part having a grain size number of 7.0 or more can be obtained, a mechanical part for an automobile having excellent static torsional strength and torsional fatigue strength can be obtained.
Further, the steel part shot-peened to the above-mentioned quenched and tempered steel part has a surface hardness of 700 Hv or more and a surface compressive residual stress of 1000 MPa or more, so in terms of static torsional strength and torsional fatigue strength, It is an even better steel part.
As described above, the present invention provides a static torsional strength, a torsional fatigue characteristic, and a torsion, which are suitable for a shaft member having a shaft portion among mechanical parts for automobiles.

It is a figure which shows the shape of a torsion test piece.

Before describing the embodiments of the present invention in detail, first, the reasons for limiting the chemical composition of the steel for induction hardening in the present invention will be explained, and then the induction hardening of the mechanical parts for automobiles made of the steel material for induction hardening in the present invention The reasons for limiting the range of each value of surface hardness, core hardness, ratio of (hardened layer depth/part radius), cross-section average hardness, and outermost surface grain size number in the tempered state will be described.

First, the reasons for limiting the chemical composition of the induction hardening steel of the present invention will be described below. In addition,% in these reasons for limitation is% by mass.

C: 0.42-0.48%
C is 0.42% or more in order to secure the surface hardness of the steel component of 600 Hv or more in the induction hardening/tempering state, and to secure the cross-sectional average hardness of the steel component of 550 Hv or more. is necessary.
However, when C exceeds 0.48%, the machinability and cold forgeability of the steel material deteriorate. Further, the fracture of the steel material during the torsion test becomes brittle fracture, and the torsional strength is rather lowered. Also, during induction hardening, quenching cracks easily occur. Therefore, C is 0.42 to 0.48%.

Si: 0.20 to 1.10%
Si is an element necessary for deoxidation, is an element effective for improving the hardenability and strength of the steel material, is an element for strengthening the grain boundary, and is an element effective for improving the torsional strength. For this purpose, Si must be 0.20% or more.
However, if Si exceeds 1.10%, the machinability deteriorates, the hardness of the ferrite matrix increases after normalizing and annealing, and cracks easily occur during cold forging.
Therefore, Si is set to 0.20 to 1.10%.

Mn: 0.70 to 0.90%
Mn is an element necessary for deoxidation, and is an element effective for improving hardenability and strength of steel materials. For this purpose, Mn needs to be 0.70% or more.
However, when Mn exceeds 0.90%, the hardness of the ferrite matrix after annealing the steel part is increased, cracks are likely to occur during cold forging, and machinability is further reduced. Further, by promoting segregation of grain boundary of embrittlement elements such as P, the static torsion strength is reduced.
Therefore, Mn is set to 0.70 to 0.90%.

P: 0.030% or less P is an element that segregates at grain boundaries to lower the torsional fatigue strength and increases the hardness of the ferrite matrix after annealing, and is an element that promotes the occurrence of cracks during cold forging. is there. Therefore, P is set to 0.030% or less.

S: 0.030% or less S is an element that improves machinability, but if a large amount of MnS is produced, cold forgeability and torsional strength are reduced. Therefore, S is set to 0.030% or less.

Cr: 0.90 to 1.20%
Cr is an element effective in improving the hardenability and strength of steel. For this purpose, Cr needs to be 0.90% or more.
However, if Cr is contained at 1.20% or more, the machinability and cold forgeability of the steel material deteriorate.
Therefore, Cr is set to 0.90 to 1.20%.

Al: 0.010 to 0.040%
Al is an element necessary for deoxidation, and also has a function of suppressing coarsening of crystal grains at the time of induction hardening heating by combining with solid solution N to form AlN. For this purpose, Al needs to be 0.010%.
However, when Al is contained in an amount of more than 0.040%, the amount of alumina-based oxide in the steel increases, and the torsional fatigue strength of the steel material decreases.
Therefore, Al is set to 0.010 to 0.040%.

N: 0.0200% or less N is an element that contributes to the suppression of crystal grain coarsening of the steel material by forming AlN and NbCN by combining with Al and Nb in steel, but N is 0.0200%. If the amount is larger, the hot workability of the steel material is deteriorated, and nitrides act as inclusions to adversely affect the torsional fatigue strength. Therefore, N is set to 0.0200% or less.

Mo: 0.10 to 0.30%
Mo is an element that is effective in improving the hardenability and strength of steel, is an element that strengthens grain boundaries and reduces brittle fracture surfaces, and is also an element that is effective in improving torsional strength. Therefore, Mo is set to 0.10% or more.
However, Mo is an element that increases the hardness of the ferrite matrix after annealing and easily causes cracks during cold forging. If Mo is more than 0.30%, the machinability decreases, and the steel material cost increases. Will increase.
Therefore, Mo is set to 0.10 to 0.30%.

Ti: 0.010 to 0.050%
Ti is an element that suppresses coarsening during carburizing and heating by forming TiC by combining with C, and Ti is an element that prevents B from becoming BN by combining with N. To this end, Ti is set to 0.010% or more.
On the other hand, when Ti is more than 0.050%, it is an element that deteriorates the machinability and cold forgeability of the steel material due to the formation of excessive TiC and TiN.
Therefore, Ti is set to 0.010 to 0.050%.

B: 0.0003 to 0.0030%
B is an element that significantly improves the hardenability of steel with a small amount of addition, and the addition of B can reduce the amount of addition of other alloying elements. Further, B is an element that strengthens the grain boundaries and reduces the brittle fracture surface, and is an element effective in improving the torsional strength. Therefore, B is 0.0003% or more.
However, even if B is contained in an amount of more than 0.0030%, the effects of improving the hardenability and strength of the steel material are saturated.
Therefore, B is set to 0.0003 to 0.0030%.

The automotive machine part of the present invention is formed into a predetermined shape of a shaft member or an automotive machine part by cold forging, hot forging, cutting or the like for carburizing and quenching steel material having the chemical composition specified in the present invention. It can be obtained by subjecting the surface of the component to induction hardening and tempering after appropriate heat treatment and the like, and further applying shot peening if necessary. Therefore, the surface hardness, core hardness, (hardened layer depth/part radius) ratio, cross-section average hardness, in the induction hardening/tempering state of the automobile machine part made of the steel material for induction hardening of the present invention, The reason for limiting each range of the crystal grain size number of the outermost surface will be described below.

Surface hardness: 600 Hv or more When the surface hardness is 600 Hv or more, the average cross-section hardness of the automobile mechanical part is increased, and the torsional strength of the automobile mechanical part is further improved. Therefore, the surface hardness is 600 Hv or more.

Core hardness (which is a non-cured layer): 350 Hv or more When the core hardness is 350 Hv or more, the average cross-sectional hardness of the automobile machine component increases, and the torsional strength of the automobile machine component improves. Therefore, the hardness of the core, which is the non-cured layer, is set to 350 Hv or higher.

(Cured layer depth/part radius) ratio: 0.5-1.0
When the ratio of (hardened layer depth/part radius) is 0.5 or more, the cross-sectional average hardness of the mechanical parts for automobiles increases, and the torsional strength of the mechanical parts for automobiles is further improved.
However, even if the ratio of (hardened layer depth/part radius) exceeds 1.0, the average cross-section hardness of the automobile machine part does not increase.
Therefore, the ratio of (hardened layer depth/part radius) is set to 0.5 to 1.0.

Cross-section average hardness: 550 Hv or more When the cross-section average hardness is 550 Hv or more, the torsional strength of automobile machine parts is improved. Therefore, the average cross-sectional hardness is set to 550 Hv or more.

Crystal grain size number of the outermost surface: 7.0 or more The larger grain size number of the surface is more effective in improving the torsional strength of the steel member, and when the grain size number is 7.0 or more, P or S The segregation amount of the grain boundary embrittlement element can be reduced, and as a result, the torsional fatigue strength of automobile machine parts can be improved. Therefore, the grain size number of the surface is set to 7.0 or more.

The surface hardness of the shot peening layer is 700 Hv or more and the compressive residual stress of the surface has a value of 1000 MPa or more. The steel material having the chemical composition of the present invention is subjected to induction hardening and tempering treatment, and further shot peening is applied. When the surface hardness is 700 Hv or more, the cross-sectional average hardness of the mechanical parts for automobiles is further increased and the torsional strength of the mechanical parts for automobiles is further improved as compared with the case where the shot peening is not performed. ..
Further, when a compressive residual stress region is formed on the surface by shot peening, crack propagation is suppressed and fracture is less likely to occur, so that the torsional fatigue strength is further improved.
Therefore, it is assumed that the surface hardness of the shot peening layer is 700 Hv or more and the residual compressive stress on the surface is 1000 MPa or more.

Here, the chemical composition of the induction hardening/tempering steel in the embodiment of the present invention will be described. Table 1 shows the sample No. The steel materials corresponding to the chemical components of the induction hardening steel of the present invention were designated as test material Nos. 9 to 11 indicate steel materials in which the shaded components deviate from the specified component range. The content of the chemical components in Table 1 is% by mass, and Fe and unavoidable impurities are excluded from the description in Table 1.

Next, Tables 2 and 3 show the characteristics of the examples and comparative examples of the test pieces that were heat-treated. In addition, the value of "No." of the examples and comparative examples in the table corresponds to "No." of the test material used for the test piece, and indicates the chemical composition of the test piece. The steel material is the sample material No. 1 in Table 1. It shows which one of 1 to 11.
Then, Table 2 shows (a) induction hardening and tempering treated induction hardening steel parts, and Table 3 shows (b) induction hardening and tempering treatments. Surface hardness, core hardness, (hardened layer depth/part radius) ratio, cross-section average hardness, surface grain size in the case of steel parts with shot peening formed by peening The numbers, surface hardness after shot peening, compressive residual stress after shot peening, static torsional strength ratio, and torsional fatigue strength ratio are described.

The materials shown in Tables 2 and 3 as examples are the test material Nos. 1, sample material No. 3 to No. Example No. 7 using No. 7 1(a), Example No. 1(b), Example No. 3(a), Example No. 3(b), Example No. 4(a), Example No. 4(b), Example No. 5(a), Example No. 5(b), Example No. 6(a), Example No. 6(b), Example 7 No. (A), Example No. 7(b).
In addition, in Tables 2 and 3, as comparative examples, the test material No. 2, test material No. 8 to No. Comparative Example No. 11 using No. 11 2(a), Comparative Example No. 2(b), Comparative Example No. 8(a), Comparative Example No. 8(b), Comparative Example No. 9(a), Comparative Example No. 9(b), Comparative Example No. 10(a), Comparative Example No. 10(b), Comparative Example No. 11(a), Comparative Example No. 11(b) is shown.
The static torsional strength ratios and the torsional fatigue strength ratios in Tables 2 and 3 are all comparative example Nos. 11 shows the ratio of the steel part 11(a) that has been subjected to the induction hardening and tempering treatment to a strength of 1.0.

(Regarding procedures related to Tables 2 and 3)
First, the test material No. Steel materials each having the chemical composition shown in Table 1 of 1 to 11 and the balance consisting of Fe and unavoidable impurities were melted in a 100 kg vacuum melting furnace, hot forged to a diameter of 45 mm, and then allowed to cool, then, After being kept at 870° C. for 1 hour as normalizing, it was air-cooled to a standard state, and further as low-temperature annealing was kept at 720° C. for 4 hours and then gradually cooled by air-cooling to obtain the obtained steel material. The torsion test piece 1 shown in FIG. 1 was processed.
Further, this torsion test piece 1 was subjected to soaking quenching by holding it at 860° C. for 0.5 hour, followed by oil cooling, then holding it at a high tempering temperature of 580° C. for 1.5 hours, and then air cooling, and further induction hardening. By tempering, the torsion test piece 1 was put into the induction-hardened/tempered state shown in Table 2(a).
Further, the test piece in the state of being subjected to shot peening treatment in a state of induction hardening/tempering to form a shot peening layer on the surface thereof was used as a torsion test piece in Table 3(b).
A static torsion/torsion fatigue test was performed on each of the obtained torsion test pieces 1 of (a) and (b).

FIG. 1 shows the shape of the torsion test piece 1. The torsion test piece 1 has a length of 150 mm and a maximum of 30 mm square, and has a through hole of φ7 mm along the axial direction and a through hole of φ4 mm in the longitudinal direction perpendicular to the axial direction at the longitudinal center portion.

The static torsion strength was measured by a hydraulic servo type torsion fatigue tester, and the value of the proportional limit was set as the static torsion strength from the data of load torque-test angle.
Further, the torsional fatigue strength ratio was a value of 10 ⁵ cycle fatigue strength under the condition of both swings and a frequency of 5 Hz.

The surface hardness of the shot peening layer portion of the torsion test piece 1 in which shot peening was further applied to the induction hardening treatment of (b) was obtained by measuring a position of 0.05 mm from the surface of the component with a load of 2.94 N. It is a value. Then, as shown in Table 3, the hardness of the shot peening layer is 700 Hv or more. Further, the compressive residual stress of the shot peening layer obtained by shot peening is a value of 1000 MPa or more, as also shown in Table 3.

The cross-sectional average hardness is a value obtained by calculating the hardness distribution from the surface to the core, then performing weighted integration corresponding to the area, and dividing by the total cross-sectional area, and is indicated by Hv. The next paragraph shows the formula for this weighted integral.

なお、ａ：捩り試験片１の半径、Ｈｖ（ｒ）：ビッカース硬さ、ｒ：中心からの距離である。

In addition, a: radius of the torsion test piece 1, Hv(r): Vickers hardness, r: distance from the center.

The crystal grain size number of the surface was calculated by the cutting method of JIS G 0551 from two fields of view photographed with an optical microscope at 400 times or 1000 times.

The compressive residual stress of the shot peening layer was measured by X-ray diffraction after driving up to 100 μm from the surface by electropolishing. Table 2 lists the highest value of compressive residual stress indicated by the measurements.

(About core hardness)
As shown in Tables 2 and 3, the hardness of the core portion is the same as in Example No. In the case of the induction hardening of (a) and the shot peening layer of (b), the value of 629 Hv is 629 Hv for Example No. 1. Regarding Example 3, in both the case of the induction hardening of (a) and the case of the shot peening layer of (b), both are 638 Hv. Regarding Example 4, in both the case of the induction hardening of (a) and the case of the shot peening layer of (b), both are 451 Hv. Regarding No. 5, both in the case of the induction hardening of (a) and in the case of the shot peening layer of (b), it was 632 Hv. Regarding No. 6, both in the case of induction hardening in (a) and in the case of shot peening layer in (b) are 642 Hv. Regarding No. 7, both in the case of the induction hardening of (a), the shot peening layer of (b), the case of the induction hardening of (a) and the shot peening layer of (b), both are 447 Hv. Therefore, in all cases, the core hardness is 350 Hv or more as specified by the present invention.
On the other hand, Comparative Example No. Regarding No. 10, both in the case of the induction hardening of (a) and in the case of the shot peening layer of (b) are 347 Hv, and the hardness of the core is lower than 350 Hv specified in the present invention.

Regarding the ratio of (hardened layer depth/part radius) The ratio of (hardened layer depth/part radius) is the same as Example No. 1(a) Induction hardening and No. In the case of the shot peening layer of FIG. 3(a) and Example No. In any of the cases of 3(b), it is 1.0, and Example No. 4(a) and Example No. In any of the cases of 4(b), it is 0.8, and the example No. 5(a) and Example No. In any of the cases of 5(b), it is 1.0, and the example No. 6(a) and Example No. In each of the cases of 6(b), it is 1.0, and the example No. 7(a) and Example No. It is 0.5 in both cases of 7(b). Therefore, all satisfy the ratio of 0.5 to 1.0 specified by the present invention.
On the other hand, in Comparative Example No. In No. 11, both in the case of the induction hardening of (a) and in the case of the shot peening layer of (b) were 0.4, which were values lower than 0.5 to 1.0 specified by the present invention. ..

(About surface grain size number)
Specimen No. Comparative example No. 2 using No. 2 2(a) and Comparative Example No. In 2(b), the grain size number of the surface is 6.8, which is lower than 7.0 defined by the present application. Therefore, Example No. 1 using the test material 1 1(a) and Example No. The torsional strength was lower than that of 1(b). In this way, No. 2(a) and No. The grain size number of 2(b) is small because the austenitizing retention time at high frequency is set to be twice as long as that under other conditions.
In addition, the sample material No. Comparative Example No. 8 using No. 8 8(a) and Comparative Example No. 8(b) has a surface grain size number of 6.9, which is lower than 7.0 defined by the present application. Therefore, Example No. 3 using the sample material 7 7(a) and Example No. The torsional strength was lower than that of 7(b). In this way, No. 8(a) and No. The reason that the grain size number of 8(b) is small is that the austenitizing holding time at high frequency is set to be twice as long as that of other conditions.
The larger the grain size number is, the more effective it is to improve the torsional strength of the steel member. When the grain size number is 7.0 or more, the segregation amount of the grain boundary embrittlement elements such as P and S can be reduced, and the torsional fatigue strength can be improved. Can be improved. Comparative Example No. 2(a) and No. 2(b), Comparative Example No. In 8(a) and 8(b), since the crystal grain size was less than 7.0, neither the static torsional strength nor the torsional fatigue strength was improved. Therefore, in Example No. 1, Example No. 3 to No. Compared to the case of No. 7, in both cases (a) and (b), the results were that static torsional strength and torsional fatigue strength were inferior.

(Regarding Comparative Example No. 9)
Specimen No. Comparative example No. 9 using No. 9 9(a), Comparative Example No. 9(b) is the sample material No. Since the C content of 5 is 0.50%, which is larger than that of the present invention, brittle fracture is likely to occur. Therefore, Comparative Example No. 9(a), Comparative Example No. In any of the cases of 9(b), Example No. 1, Example No. 3 to No. As a result, the static torsional strength and the torsional fatigue strength are inferior to those in any of 7 (a) and (b).

(Regarding Comparative Example No. 10)
Specimen No. Comparative example No. 10 using No. 10(a), Comparative Example No. No. 10(b) is the sample material No. Since the C content of 10 is 0.35%, which is smaller than that of the present invention, the surface hardness is low and the core hardness is also low. The average cross-section hardness is also lower than 523 Hv and 550 Hv, and the torsional strength is not improved. Therefore, Comparative Example No. 10(a), Comparative Example No. In any of the cases of 10(b), the example No. 1, Example No. 3 to No. As a result, the static torsional strength and the torsional fatigue strength are inferior to those in any of 7 (a) and (b).

(Regarding Comparative Example No. 11)
Specimen No. Comparative Example No. 11 using No. 11 11(a), Comparative Example No. 11(b) is the sample material No. Since the C content of 11 is 0.40%, which is smaller than that of the present invention, the surface hardness is low. And the average hardness of the cross section is lower than 532 Hv and 550 Hv, and the torsional strength is not improved. Therefore, Comparative Example No. 11(a), Comparative Example No. In any of the cases of 11(b), the example No. 1, Example No. 3 to No. As a result, the static torsional strength and the torsional fatigue strength are inferior to those in any of 7 (a) and (b).

1 Torsion test piece 2 Through hole of φ7mm 3 Through hole of φ4mm

Claims (5)

As chemical components, in mass%, C: 0.42 to 0.48%, Si: 0.20 to 1.10%, Mn: 0.70 to 0.90%, P: 0.030% or less, S : Induction hardening containing 0.030% or less, Cr: 0.90 to 1.20%, Al: 0.010 to 0.040%, N: 0.0200% or less, with the balance being Fe and inevitable impurities. A mechanical part for automobiles made of steel
In the induction-hardened/tempered state, the surface hardness is 600 Hv or more, the core hardness is 350 Hv or more, the ratio of (hardened layer depth/part radius) is 0.5 to 1.0, and is expressed by the following mathematical formula. A machine component for automobiles, which has an average cross-section hardness of 550 Hv or more and a grain size number of the outermost surface of 7.0 or more.

a: radius of test piece, Hv(r): Vickers hardness, r: distance from center In addition to the chemical composition according to claim 1, an automotive machine part made of induction hardening steel containing Mo: 0.10 to 0.30% by mass and the balance Fe and unavoidable impurities. hand,
In the induction-hardened/tempered state, the surface hardness is 600 Hv or more, the core hardness is 350 Hv or more, the ratio of (hardened layer depth/part radius) is 0.5 to 1.0, and is expressed by the following mathematical formula. A machine component for automobiles, which has an average cross-section hardness of 550 Hv or more and a grain size number of the outermost surface of 7.0 or more.

a: radius of test piece, Hv(r): Vickers hardness, r: distance from center In addition to the chemical component of Claim 1 or Claim 2, it contains Ti:0.010-0.050% and B:0.0003-0.0030% by mass %, and the balance is Fe and unavoidable. A machine part for an automobile made of induction hardening steel made of impurities,
In the induction-hardened/tempered state, the surface hardness is 600 Hv or more, the core hardness is 350 Hv or more, the ratio of (hardened layer depth/part radius) is 0.5 to 1.0, and is expressed by the following mathematical formula. A machine component for automobiles, which has an average cross-section hardness of 550 Hv or more and a grain size number of the outermost surface of 7.0 or more.

a: radius of test piece, Hv(r): Vickers hardness, r: distance from center The mechanical parts for automobiles have a shot peening layer on the mechanical parts for automobiles that have been induction hardened and tempered. The surface hardness of the shot peening layer is 700 Hv or more, and the compressive residual stress of the surface is 1000 MPa or more. It has, The mechanical component for motor vehicles of any one of Claim 1- Claim 3 characterized by the above-mentioned. The mechanical part for vehicles has a shaft member, The mechanical part for vehicles of any one of Claims 1-4 characterized by the above-mentioned.

Images